This invention relates to the art of detecting a liquid level, which is particularly useful in detecting leaks in liquid storage tanks. More particularly, this invention relates to the art of detecting leaks in tanks used for storing hydrocarbon-based fuels.
To detect a loss of 0.02 gallons of liquid per hour from a storage tank capable of storing 1,000 gallons or more, e.g., an undergound gasoline storage tank, by determining changes in the liquid level in the tank can be very difficult. By way of illustrating this difficulty, consider a cylindrical tank oriented on its side having a length of 21.3 feet and a diameter of 8 feet. When half full of liquid, such a tank would contain about 4,000 gallons. If one gallon were removed from such a tank, assuming all other relevant variables to be discussed hereinafter were held constant, the level of the liquid measured relative to the lowest point of the tank would change from 4 feet to 3.9992 feet, i.e., a change in the liquid level of 7.85.times.10.sup.-4 feet. Accurate detection and measurement of such a minute change in the liquid level in an underground storage tank is extremely difficult.
Furthermore, even when such a change in a liquid level can be accurately and reliably measured, the change may not always be due to a loss of liquid. For example, even if there were no leaks in such a tank, a change in temperature of the liquid, e.g., gasoline, of 0.25.degree. F. would cause a density change of 0.01139 lb/ft.sup.3 cubic feet per pound, resulting in a change in the observed liquid level of 7.85.times.10.sup.-4 feet. Thus, a decrease in temperature of only 0.25.degree. F. would change the gasoline level in such a tank by the same amount as would the removal of one gallon of gasoline.
Consider also the impact of evaporation. Assume, for example, that such a tank were half filled with gasoline at 70.degree. C. and that the air at about one atmosphere above this gasoline in the tank had substantially no gasoline vapor. The level of liquid would change by 7.38.times.10.sup.-3 feet as a result of the vapor above the liquid changing from containing substantially no gasoline vapors to being saturated in gasoline vapors. Such a decrease in the gasoline level would be approximately the same as that caused by the removal of 9.40 gallons of gasoline.
Accurate detection of the loss of liquid from storage tanks has important environmental and safety consequences, particularly in storage tanks used for storing hydrocarbon-based fuel. Even relatively small losses from such tanks can present a safety hazard and pollution concern. As a result, there are a number of leak detecting devices known in the art that are designed to detect and measure leaks. Because gasoline service stations usually employ several such tanks disposed underground, such devices often are capable of detecting leaks where visual perception of the tank is impossible.
Three objectives of the design of such leak detection devices are that they be (1) easy to use, (2) highly reliable, and (3) safe. Testing for leaks occurs throughout the world and is performed by a large number of persons of varying degrees of skill often in unpredictable and adverse environmental circumstances. Replacement of an underground tank is a costly and time-consuming task, but, if required, must be done quickly, especially when the stored liquid is a hydrocarbon-based fuel such as gasoline, which is highly volatile.
An apparatus and method capable of detecting and measuring such minute changes in the level of liquid is described in U.S. patent application Ser. No. 208,608 which has been assigned to the assignee of the present invention. Briefly, one embodiment of that invention is an apparatus capable of detecting extremely small changes in the relative distance between a float means and both a signal source and a detector. The float means floats in a storage tank containing a liquid. The signal source and the detector remain in a fixed position relative to the storage tank. Changes in the height of the float means, which can be detected by the signal source and detector, indicate changes in the level of the liquid stored therein. Detection and precise measurement of the changes in the height of the float means, in the manner briefly described below and more fully described in U.S. patent application Ser. No. 208,608, which is expressly incorporated herein by reference, permit accurate measurement of leaks in the storage tank.
The apparatus of that invention comprises in combination: a signal means for providing or emitting a signal having a characteristic; a detector means for detecting a modulated signal for producing a response signal directly related to the characteristic; a support means for positioning the signal means and the detector means; and a float means comprising a means for floating at a desired depth in a selected liquid and a reservoir means for holding a measuring medium. The detector means and the signal means are fixedly connectable to the support means. The float means is movably connectable of the support means. Therefore, the measuring medium held in the reservoir of the float means is permitted to move relative to the signal means and the detector means. The signal, as emitted by the signal means, is modulated by the measuring medium. The detector means is oriented to detect the modulated signal. The amount by which the emitted signal is modulated by the measuring medium to produce a modulated signal varies with changes in the location of the float means.
An example of a signal means is a conventional light bulb.
An example of a detector means is a photo resistor which, for example, uses cadmium sulfide.
An example of a signal is electromagnetic radiation such as would be emitted by a conventional light bulb.
An example of a characteristic of a signal is intensity.
An example of a float means is a hollow, elongate tube of a predetermined weight having a closed end at a desired depth in the liquid. The float means may, for example, be constructed of a plurality of sections of tubing, such as 2-1/2 inch diameter aluminum tubing with the bottom section being a closure means for forming the closed end of the tube. The sections may be of convenient lengths and may be interconnected by any suitable means such as being threaded into one another.
An example of a measuring medium is a liquid having a k value defined in relation to Beer's Law in the range of about 0.1 to 1,000 reciprocal centimeters. Preferably, the k value of the measuring medium has a value in the range of about 20 to about 200 reciprocal centimeters.
Thus, for example, a float means having a reservoir holding a measuring medium is positioned to float at a desired depth in a liquid while being movably connected to a support means. A photo resistor and a conventional light bulb are fixedly connected to the support means, which, in turn, is fixed relative to the storage tank. As a consequence, the float means and its measuring medium are allowed to change position relative to the light bulb and photo resistor as the level of liquid in the tank changes. The light bulb emits electromagnetic radiation having a specific intensity. The photo resistor is positioned in the reservoir and detects the intensity of the light as modulated by the measuring medium. A change in location or height of the float means results in a change in the amount of measuring medium between the photo resistor and the light bulb, thus modulating the electromagnetic radiation detected by the photo resistor. Changes in the resistance of the photo resistor indicate changes in the location of the float means.
An improvement to this method involves locating the float means at a predetermined depth calculated such that temperature variations of the liquid in the storage tank do not change the depth at which the float means floats freely. In other words, a substantially temperature invariant floating position is maintained by the float means. The approximate depth at which the float means is maintained is calculated by dividing the volume of the selected liquid in which the float means is floating by the free surface area of the selected liquid. A still further improvement to the method involves saturation of the vapor space of the storage tank with the liquid therein so as to minimize evaporation of the stored liquid during measurement of the location of the float means.
The float means of the aforementioned apparatus contains weights of predetermined weight in the bottom thereof to maintain the float means in a substantially vertical orientation within the liquid in the storage tank. Use of specifically calculated weights permits floating the float means at a desired depth on the selected liquid. The float means includes a reservoir means for holding the measuring medium. Advantageously, the float means is constructed from elongated tubing such as 2-1/2 inch diameter aluminum tubing. Because the buoyancy factors acting on the float means can be calculated, further calculations can be used to determine the proper amount of weights to be placed in the float means to maintain it at the desired depth. Once these calculations are performed and the proper amount of weights is added, the reservoir is inserted into the top of the float means prior to insertion of the assembly into the storage tank for measurement.
The apparatus described is particularly useful in detecting leaks in tanks used for hydrocarbon-based fuels such as gasoline. Such tanks are often large underground horizontal cylindrical tanks where visual examination of the walls thereof is not possible. During testing of the apparatus, it is occasionally observed that the data would indicate that the float means would move relatively further downward in the storage tank under circumstances in which the level of the liquid stored therein had not changed. Thus, while the data recorded by the apparatus would indicate the presence of a leak, in fact, no leak existed. While such a condition would not detract from the safety of the leak detector device or its ability to detect actual leaks, it would on occasion produce data which was falsely indicative of leaks.
It was not known whether the false data indicating the presence of a leak was caused by a reduction in the volume of the liquid due to circumstances other than a leak such as due to changes in its density, by failures in the test data recording apparatus, or by other causes such as an unexplainable tendency of the float means to slowly sink in the liquid. As can be readily appreciated, any factors tending to cause minute variations in the height of the float means over time, other than an actual change in the level of the liquid, would tend to indicate changes in the level of the liquid stored in the tank when, in fact, no such changes had occurred. Even the most minute increase in the depth at which the float means floats, which changes the relative height of the top of the float means and the reservoir mounted thereon, tends to indicate, falsely, substantial leakage.
It was discovered that condensation of vapors within the float means increased the weight of the float means, resulting in small but detectable changes in the position of the float means. These changes in depth of the float means were found to be responsible for the false data indicating leaks in the storage tank. The existence of even just a few drops of condensate on the inside of the float means was sufficient to cause the data to suggest false leaks. It is believed that this problem is particularly acute where the vapor space above the stored liquid is saturated with fluid vapors, as is practiced in one embodiment of the invention disclosed in U.S. patent application Ser. No. 208,608. Saturation of the vapor space may be accomplished by extending a suction line to the bottom of the tank, as, for example, within approximately 12 inches thereof, and by pulling some of the liquid stored in the tank up through the suction line by means of a self-priming pump and forcing the liquid through a spray head located in the vapor space of the tank. Of course, where the stored liquid is a hydrocarbon-based fuel, the pump should be explosion-proof. Preferably, the spray head is located approximately three to six inches below the top of the tank and is of the type that distributes the liquid in a fan pattern. To ensure complete saturation, the spray head may be rotated during the saturation process. Saturation may also be accomplished by spreading a thin layer of the liquid on the exposed surfaces within the vapor space above the liquid. It may also be accomplished by bubbling gas vapors through the liquid stored in the tank so that the saturated gas vapors displace the unsaturated vapors above the liquid. While saturation of the vapor space prevents the problem encountered when the stored liquid evaporates, it is believed to enhance the possibility of condensation forming on the inside of the float means. Furthermore, when the liquid stored is a hydrocarbon-based fuel such as gasoline, the vapors thereof are readily susceptible to condensation. The present invention eliminates the problem of condensation within the float means through the use of a deformable plug capable of forming a vapor tight seal at the upper, open portion of the float means. A plug is used to seal the float after the proper amount of weights have been added to it. Preferably, the deformable plug comprises a pressure deformable sealing ring. An example of material suitable for use in a plug of this invention is rubber, so that it may be removed and resealed within the float means. In this way, fluid vapors are prevented from entering the float means and condensing therein.
Thus, it is an object of the present invention to provide an improved apparatus and method for detecting leaks in liquid storage tanks.
It is a further object of the present invention to provide a simple and easy to use apparatus and method capable of detecting small leaks, e.g., as little as 0.02 gallons per hour in a tank capable of storing 1,000 gallons or more, without giving false readings indicating a leak.
These and other objects, features and advantages of the present invention will become apparent in the following detailed description.